text:
Does adding a motorized fan to a hot air balloon change its class as an aircraft?
I have seen fans like these for use in ballooning and glider propulsion.
Does adding one of these change the definition of the flight vehicle?  e.g. is it No longer a balloon, but now an aircraft?  Does only the flight elevation and proximity to a controlled airspace matter — or does the capacity of the craft have an impact?
I suppose another dimension of this question regards if licensure changes in such case.

Yes, a hot air balloon would change class, if you are adding an engine and propeller to provide thrust. With powered flight comes the need to provide some form of flight control; at this point, you have an airship, albeit a thermal airship.
According to the FAA's Airship Design Criteria, and much like @voretaq7's citation in another answer:

An airship is an engine-driven, lighter-than-air aircraft, that can be steered.

The FAA has a whole section on Airships, for further reading.

So, does the class legally or only practically change?  It's legal to hold a PPL for gliders and fly a powered glider with no additional training.  A powered glider is not "Airplane Single Engine Land" simply because it has an engine.  It's still a glider and requires no medical or additional training.  What is the mechanism that formally and automatically converts a baloon to an airship by simply adding a fan?

Well, a balloon already is an aircraft, specifically:

A balloon is a lighter-than-air aircraft that is not engine driven, and that sustains flight through the use of either gas buoyancy or an airborne heater.

So if you were to attach engine-driven propellers to your balloon it would no longer be a "balloon". It would start to sound a lot more like another kind of aircraft -- an airship:

An airship is an engine-driven, lighter than air vehicle that can be controlled in flight.

So from a licensing standpoint there would appear to be two major considerations: First the pilot of such a chimera would need to be licensed to act as pilot in command of an airship (not a balloon), and second the modification itself would have to go through appropriate regulatory channels, since it is changing the fundamental nature of the aircraft in a way that would almost certainly attract some scrutiny.

Specifically regarding the fans/propellers you're talking about, it looks like those are intended for use as inflation fans to help you out when you're setting a hot-air balloon up - once you've got the envelope inflated and the heater is doing its job you don't really need the fan anymore, and you would leave it with your ground crew.
As far as I can tell they aren't intended for use as engines for moving and steering a balloon - I suppose you could carry one aloft with you if you were feeling bold, but I'm not sure what the impact of doing so would be from a regulatory standpoint when it's not permanently attached to the balloon, and I'm not sure how effective they would be as propulsion with only a few horsepower behind them.

There's a reason they're called inflation fans... The propellers are just for replacing the built-in ones for the inflation fans, not propellers as in the ones for engines.


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What is this star shaped symbol on the tarmac?
This symbol is on the tarmac at Ramona, CA airport.  I've seen it at other small GA airports from Google Earth but I don't know what it is.  Can someone please explain?

That is a Compass Rose, painted on a Compass Calibration Pad. It's used to mark a location on the airport surface that is suitable for calibrating the compass of an aircraft.
Here's one in a slightly different style:

and a more basic one from an FAA diagram:

More information on the requirements for the compass rose / compass pad can be found in AC 150/5300-13, in Appendix 6.

Fun bonus fact: The compass rose in your photo was painted by The Ninety-Nines - they volunteer to do compass rose paintings at a lot of GA airports, and you can recognize their work by the 99s logo in the center of the compass rose.

Related: Marking the way.

@voretaq7  nice trivia.  The 99s logo confused me because the runway is 9-27 and I thought it might be related to the runway heading.

Magnetic north drifts around fairly significantly - are these repainted on a regular basis?

@meowsqueak Per the advisory circular: Magnetic surveys of existing compass calibration pads must be performed at regular intervals of 5 years or less. … Pads not resurveyed after 5 years or after nearby construction should not be used. - Whether this is actually done in practice varies, so it is prudent to check with the airport to determine the last time the compass pad was properly surveyed.

@voretaq7 It seems odd that the compass rose is aligned to magnetic north instead of true North.  Mag north does have to be re-calibrated which seems like a lot of work if the rose would have to be re-painted.  I realize the variation changes slowly but if it was aligned to true north it wouldn't have to be re-calibrated, except maybe every 50,000 years or so.

@PJNoes Remember we're calibrating a magnetic compass: If the compass rose were aligned to True North they would have to paint or other wise mark (with non-magnetic materials) the magnetic variation with the same level of precision - that means they still have to do the periodic surveys, even without regular repaints. They also need to accurately point the aircraft's nose (and its magnetic compass) at the appropriate angles accounting for that variation, which isn't easy without painted markings - at that point they may as well use a calibrated master sight compass rather than a compass rose.

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text:
What is the difference between "closed traffic" vs "stay in the pattern" request?
My instructor made "request right closed traffic..." call before practicing a series of landings. Just wondering, what is the difference between this request and a "stay in the pattern..." request? Which one should I use when?

Actually, "stay in the pattern" is non-standard phraseology and shouldn't be used, even though it means the same thing as "closed traffic".
Specifying the direction isn't required but it is a good idea to ensure that everybody is thinking the same thing.

The answer is clear, thanks, but I learned "stay in the pattern" phrase from Comm1 VFR radio-simulator. This should be popular computer program to train communication skills, does your answer mean I cannot trust/use this software for improve my radio-communication skills?

@barmatat "Just because it's popular doesn't mean it's right." (see the ever popular but blatantly incorrect expression "any traffic in the area please advise"). When in doubt refer to the official source

@barmatat To the contrary, I think that software like that is very useful when you are first learning.  However, that doesn't mean that everything is perfect and correct.  As with many things in aviation, "trust but verify".  :)

"stay in the pattern" may be non-standard, but "remaining in the pattern" is found in the AIM under UNICOM phraseologies - and therefore is standard, at least for non-towered airports.

The recommended phraseology depends on the airport. If it's tower controlled then you're encouraged to use ATC's phraseology per JO 7110.65V
(3−10−11):

PHRASEOLOGY− LEFT/RIGHT (if required) CLOSED TRAFFIC APPROVED. REPORT
  (position if required),

For uncontrolled airports we see this in the AIM (4-1-5):

Recommended UNICOM phraseologies:
(b) Outbound PHRASEOLOGY− FREDERICK TRAFFIC CESSNA EIGHT ZERO ONE
  TANGO FOXTROT DEPARTING RUNWAY ONE NINER. “REMAINING IN THE
  PATTERN” OR “DEPARTING THE PATTERN TO THE (direction) (as
  appropriate)” FREDERICK.

Successive operations involving takeoffs and landings or low approaches where the aircraft does not exit the traffic pattern are defined in both JO 7110.65V and the AIM as Closed Traffic. However, the recommended phraseology for UNICOM is to say "remaining in the pattern", whereas for towered operations ATC will always use "Closed Traffic". In practice, pilots and ATC will know what you mean either way.

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text:
How much power is used by cabin lighting systems during flight?
How much power is used by cabin lighting systems during flight when all the lights are ON?

Almost all lighting systems installed today use Light Emitting Diodes for weight savings, reduced power use and less maintenance. Compared to other aircraft systems, their power use is pretty insignificant. For the reading lights, I found the following quote:

Passenger reading lights in halogen technology use 11 watts, while the
  latest LED products use only one watt. Source

The actual ceiling ceiling lighting power consumption seems more difficult to find. SAAB in a brochure for converting fluorescent to LED lights for their SAAB 340 & 2000 models states:

Power consumption is reduced from 8 watts per 12 inch tube to just 5,1
  watts.

Emteq suggests savings of around 15% for LED compared to fluorescent lights, so SAAB's suggestion would be a bit more. This manufacturer quotes 10W for a 16-inch (40cm) LED strip running off the 28V aircraft DC system. Needless to say, power consumption depends on the wattage of light installed.

I cannot find Mike Foxtrot's answer so this might be duplication. The power consumed is the power used by each lamp multiplied by the number of lamps. Every type of aircraft is likely to have its own type of lamp & its own number of lamps. Also, cabin lamps can be dimmed & reading lamps are a variable load.
LEDs are getting more powerful (light output/energy consumed) so fewer lamps are needed, but this comes at the price of more heat generated; this requires heatsinks. So the designer will select lamps on the basis of light output, area to be illuminated, power consumption, heat output & heatsink size.
So a definitive answer to your question could only apply to one type of aircraft.

I always thought LEDs were more efficient than incandescents; they're working on 100w equivalents for home light fixtures that use just 20w or so. The flash LED in your phone uses only one or two watts, and those are so commonly used as flashlights it's not even considered a hack anymore.

LEDs are more efficient than incandescent lights. Heatsinks are needed due to power density - the physically tiny LED dies can't dissipate they generate alone (even though it's less than what an incandescent bulb of the same light output produces) so they need heatsinks to prevent damage.

According to this link, an Airbus A-321 has around 100 LED units for cabin lighting which have 45 individual diodes each. Do you have 4500 diodes in the cabin in a single aisle plane, which means at 0.05W per LED estimation, the cabin lighting would require 225 W of power.
p.s. This does not include reading lamps (most probably) and only the aisle lighting.


text:
What is the difference between a concourse and a pier?
Both are narrow buildings with aircraft parked on both sides, so what is the main difference?

(image source)

And here you have the page on wikipedia: https://en.m.wikipedia.org/wiki/Airport_terminal

To me the difference in the image is quite obvious: concourses are connected with a bridge or tunnel while piers extend from the main building. But whether that is official classification I don't know—and I don't think it is, because I don't see a point in having official classification in the first place.

As pointed out on Wikipedia, there's quite a bit of flexibility on the terms. A concourse is not necessarily remote (basically, each of the areas of a terminal is usually called a concourse, whether remote or adjacent to the check-in/arrivals facilities). A remote concourse may or may not be called a satellite.

In terminal designs, the main difference between a pier and a concourse is that a pier is connected by terminal buildings on one side, but a concourse has separate terminal buildings connected by transportation either under or over ground.
In practice, the terms "terminal" and "concourse" are often used interchangeably, with "satellite" being used in that specific case, and "piers" being referred to as terminals or concourses.
Simple
Small terminal with a few gates.
Linear
Longer than a simple terminal, with gates on one side. May be curved or straight.
Concourses
Standard terminal with gates on one side, with additional linear terminals connected underground or by buses (occasionally by a bridge), each with gates all around. Each is generally designated as a different concourse.
Standard
A main terminal area, with long extensions having gates down one side. May be designated into different terminals depending on the size.
Pier
Like concourses, extensions with gates down both sides, but connected on one end by additional buildings. Each pier may be designated as a separate terminal.
Satellite
A main terminal area connected to one or more smaller terminal buildings with gates all around. Generally connected underground, occasionally by a bridge. Each satellite is generally designated a different terminal.

Just wondering, do you have a source for more info, or is this personal knowledge?

@digitgopher Based on the image above and the Wikipedia article along with personal knowledge... I'll see if I can find any good sources.

"Concourse" is not really a specific concept, but denotes some type of hangar remote from the main terminal. Such concourse has been required to extend the original integrated terminal when traffic has grown.

The building on the right is the main terminal, a standard simple terminal. The two concourses on the left are remote piers (facing rows) to extend the original terminal.
On the other hand a pier is the well known configuration used originally to moor boats, with two facing rows fed by a single central path:

Marina of La Rochelle, France
A main terminal can be a linear or convex (gates) unit. It can be extended by concourses of varying density, infrastructure type, or connection.

Source: Airport Engineering: Planning, Design and Development of 21st Century Airports
In the figure above a terminal can be:

An integrated terminal: linear (2c) or convex (3c).
Extended with connected piers (2c) or connected satellites (2b).
Extended with unconnected piers (3c) or unconnected satellites (3b).
Limited to the main building extended by apron (open air) stands (2d, 3d), requiring an interface between the protected area (terminal) and the free area used before boarding (apron). This interface is usually connected to a bus.

Today the trend is to plan an airport and its future extensions as an integrated building, and to start building only what is required at the time of the opening. This removes the complexity of remote extensions: Security, boarding interface, baggage processing, access to shops... maintaining passengers into a closed volume until they have boarded their plane.

As used in the wild, "concourse" seems to refer to a group of gates with a common access route, i.e. it is about passenger nnavigatio. Rather than say gates 1-20 are to the left and 21-40 are to the right, it is simpler to say "A" concourse (gates A1-A20) to the left and "B" concourse (gates B1-B20, or rarely B21-B40) to the right. It doesn't matter whether those concourses are piers, satellites, remote, local, etc.


text:
Is there an "SR-72" under development?
The Extreme Tech website reports that

Lockheed Martin’s Skunk Works has confirmed that it is developing the SR-72 spy plane. The successor to the SR-71 Blackbird, which was capable of Mach 3.5, the SR-72 will be a hypersonic unmanned aircraft capable of Mach 6, or just over 4,500 mph.

The article goes on to say it will be unmanned, with the full scale vehicle built & tested by 2030.
It's not that I would ever doubt Extreme Tech or the author of this story (cough, cough), but it seems odd that news of an SR-72 project isn't being far more widely reported.
Are there any other sources reporting this new project, or is Extreme Tech the only one?

https://en.wikipedia.org/wiki/Lockheed_Martin_SR-72

Related: http://aviation.stackexchange.com/questions/20869/what-advantages-would-a-triangular-shaped-spy-plane-have-over-a-an-sr-71

The article is a few years old by the way. So nobody would be reporting this right now. (Not a comment about the truth or otherwise of the story though.)

How is whether other sources have reported this "opinion-based?" Clearly they have, including Lockheed Martin itself.

dont forget, it might be secret? jokes aside.

Yes, the SR-72 project has indeed been officially announced. Lockheed confirmed it to Aviation Week back in 2013. Apparently it actually did get quite a bit of attention at the time, enough to actually cause Aviation Week's servers to go down for a while after the announcement.
The source quoted in the question is correct that Lockheed's announcement did confirm that the SR-72 would be hypersonic:

But now Lockheed Martin believes it has the answer. “The Skunk Works has been working with Aerojet Rocketdyne for the past seven years to develop a method to integrate an off-the-shelf turbine with a scramjet to power the aircraft from standstill to Mach 6 plus,” says Brad Leland, portfolio manager for air-breathing hypersonic technologies.

Also, from Lockheed Martin's own website, confirming both the Mach 6 capability and the intention for it having at least the option of flying unmanned:

Envisioned as an unmanned aircraft, the SR-72 would fly at speeds up to Mach 6, or six times the speed of sound.

According to Lockheed, a technology demonstrator aircraft may be flying by 2018:

Lockheed has run scaled tests on components. “The next step would be to put it through a series of tests or critical demonstrations,” Leland says. “We are ready for those critical demonstrations, and we could be ready to do such a demonstration aircraft in 2018. That would be the beginning of building and running complete critical demonstrations. As of now, there are no technologies to be invented. We are ready to proceed—the only thing holding us back is the perception that [hypersonics] is always expensive, large and exotic.”

Here's a rendering of the proposed design from the SR-72 Wikipedia page:

Source: Wikipedia, which apparently took it from Lockheed's website.
Lockheed was recently awarded a grant from NASA to study the viability of their proposed combination turbine/ramjet engine solution for the SR-72.

Task Order No.NNC15TA03T provides for a parametric design study to establish the viability of a Turbine Based Combined Cycle (TBCC) Propulsion system consisting of integrating several combinations of near-term turbine engine solutions and a very low Mach ignition Dual Mode RamJet (DMRJ) in the SR-72 vehicle concept. Task Order NNC15TA03T is issued under Contract NNC10BA08B on a firm fixed price basis. Firm fixed price is $892,292.

Lockheed Martin is developing a lot of stuff
That's kind of the point of the skunkworks - do lots of R&D in lots of areas (even if you don't finish most/all projects) so that you're ready to respond quickly to whatever challenge comes up next.
That said, an SR-72 is pretty unlikely - who needs a high speed, stealthy recon aircraft when you have satellites?
It is in development/under research, as a response to potential anti-satellite weapons. But it is almost certainly a top-secret, classified project....why would it be widely reported on? There will be very little information available, and any reports would likely be shut down rapidly at source if they contained much more than speculation. The project exists, we know that, but we're unlikely to get any more information, and therefore there's nothing much to say in a report

Your last two sentences seem to be quite contradicted by reirab's answer. There probably aren't too many accurate and precise specifications available, but the general info seems to be readily available. I agree completely with everything else you've said.

A very good point, adjusted to take that into account.

By the way, as far as I know, it's not legal in the U.S. for the government to stop publication of reports, even if they did stem from highly classified information. It's only a crime for people who have a security clearance to release the classified information. People who have received such leaks, however, have no legal restriction from continuing to disseminate it, per legal interpretation of the First Amendment. That's how every news outlet in the country was able to quote Top Secret information leaked by Bradley Manning and Edward Snowden.

I wasn't saying that being classified means the publication will be stopped... simply that there will be very little information available to report on. Certainly if there was a leak there would be reports.

Another reason planes are still favoured is they're less predictable than satellites; satellites currently only make two passes a day at most, unless you can co-ordinate use of several for shorter intervals in the coverage.

@Andy You can also get way better resolution photography from an airplane than from a satellite.

@Andy: That's not necessarily true. At low inclinations and low height, you could have more than a dozen passes. The major reason that's not done is that there simply aren't any "interesting" countries around the equator.


text:
Could New York build a new mega airport?
Both New York and London have insufficient air traffic capacity. One option for London is to build a new mega airport to replace LHR. However, this option is not discussed much in NY. Would it be feasible to build a new mega airport to serve New York?

I think I'd challenge your assumption of insufficient air traffic capacity.  You could equally well look at it as too many planes wanting to land, or (for NYC at least) insufficient passenger rail service.
Beyond that, have you looked at land prices in the vicinity of NYC or London?  Perhaps in some countries you can just seize land to build airports, over the objections of the people living there.  In the US (and presumably the UK, though I'm not certain), it's much more difficult to obtain, and should properly involve paying a fair market price - after long and expensive legal battles.
Then you have to consider trip duration.  It's not just time spent flying, but time getting to and from the airport.  In NYC, it's IIRC about a 15 minute bus ride from LaGuardia to Manhattan, and I think not much more from JFK.  Put the airport way out in the country, and people would spend an hour or two just getting there, negating the speed advantage of air travel for short & medium length flights.

Two closer airport is good for local but big troubles for connectors,and legal battles existed for airport expansion too---and you have to pay more fair value. I prefer a concentrated huge internatiobal hub, no matter expansion or new built, because it provides better experiences for connectors and also for airlines.

PS: Gatwick is located in sururban area and thames hub is in rural area. Personally I prefer 4runways (not 3) in hwathrow, but heathrow is surrounded by residential housing

@Him: But you are presumably not one of the people living on or near the proposed airport, or paying for it, so your preferences don't carry that much weight.

Due to regulations, 2 runways in an airport are not answering less demand in an airport than 2 times one runway. So there is a loss of efficiency when 2 runways are built, that depends strongly of their relative orientation and their separation.
That means, that creating such a big airport will start requesting a massive size of land to have enough separation between airplanes and/or inefficient use of runways. You can easily finalize having flights of 1 hours and trips around that airport of 30 minutes.
Due to this limitation, and the potential cost of the land (usually very expensive close to the city) eliminates economies of scale, making more economically efficient to create 2 airports than creating a single big one.
Finally is a balance between several economical factors, but you can see that London, Paris, Berlin and New York are currently served by several airports.
Taking that information to answer your questions:

Actually London is not thinking in creating a big airport, they are trying to expand LHR or Gatwick
Concerning NY, creating a big airport to replace La Guardia, Newark and JFK will be a very big airport very far from the city center and very likely to not be efficient.

I don't understand what you are saying in your first paragraph. You have a good point though. Lots of large cities are served by multiple airports, for both historic and economic reasons.

@fooot, sorry for that. What I am trying to mean is that depending on how the runways are positioned relatively they will be allowed different modes of operations which create restrictions to use simultaneously the runways (or you move them away during the construction). So the capacity (number of flights) of a 2 runways is not the double of one runway isolated. Modes of operation are here: http://www.skybrary.aero/index.php/Parallel_Runway_Operation

@Trebia Project. At least mega airport was one opinion discussed by the airport commission in UK, even though it is rejected. But there is no such discussion in NY.

@Trebia Project Moreover several large cities including Mexico city , Istanbul and Beijing are building new mega airport instead of expansion.

In addition to the reasons listed above, they would be creating the world's new busiest airport.

@Him Mexico and Istanbul airports are planned for 6 runawys, that's not a "megairport".  O'hara in Chicago has got 7 runways and has another airport (Midway). Beijing (if the full project is constructured) will have 9 runways, which can be described as a megairport. That's right. In this case probably the economical criteria is not the main criteria. Probably prestige of having the biggest airport will be part of the decision although not economically as efficient as 2 airports (Maglev in Shangai is a similar example of prestige over economy)

@TrebiaProject. "mega" airport is more likely to be determined by size but not number of runways. Mexico city is half larger than Ohare and Istanbul is double size compared to Dallas.

@Him how you measure size? Notice that passengers, size of the terminal and runways are not independent parameters.

@trebia They are independent but runway capacity is much more than you expected, two closed parallel runways like dubai can serve more than 400k aircraft movement annual. Therefore 6 runways is enough to serve 3 major airport in ny. If the runway is widely seperated each runway can serve more flight, the london thames hub airport which serve 100k passengers, have 4 runways and 33km2 size only


text:
Why do some commercial aircraft have no landing gear cover?
The B737 has no cover on its landing gear, as may be seen in this picture. Doesn't this increase the air drag?

Almost all other airliners have doors that close off the rubber of the wheels, for example this B787.

Why doesn't the B737 have a cover on its landing gear?

@anshabhi Thanks for the edits

The 737's hubcap is its landing gear door.

Yes, it does increase drag.  Anything that is not smooth, where the air flow will be disturbed, will generate drag.
The 737 was originally designed only for short haul and space is at a premium.  Adding doors and all the associated mechanisms adds complexity, weight, additional maintenance requirements and more possibilities for failure.
Boeing have had many opportunities to add doors and have chosen not to; so they, and their customers, do not see the drag penalty as more expensive than the extra weight and maintenance.


text:
Can an airliner fly out of a downburst?
Many aircraft accidents have been caused by downbursts. A downburst is basically a strong downward wind that forces the aircraft toward the ground and causes it to crash. Is it possible for an airliner to escape the downburst and land safely with this much force pushing down on an aircraft?
Here is a downburst illustration: Source:(www.mikesmithenterprisesblog.com)

power, power, power!

First take a look http://www.erh.noaa.gov/cae/svrwx/downburst.htm for info on downbursts.
An aircraft encountering a microburst (the more localized form of a downburst and that which airline crews train for) can counter it in two basic ways:

establish, if possible, a climb rate through the descending air that is greater than the downward vertical speed of the air.
fly through the column of descending air to air that is not descending or is at least descending at a lesser rate

And, of course, do this without stalling the airplane, especially as you come out of the burst. A practical scenario is that your max climb rate through the descending air can at least reduce your descent rate toward the ground enough that you will be able to transit the burst before hitting the ground.
I was never in a full blown microburst for real, but I practiced recovering from them many times in simulators. For 747-100/200 aircraft in the 1990s the procedure was:

thrust levers all the way forward
raise the nose until you get the stick shaker
fly the aircraft straight ahead while keeping the speed solidly in the stick shaker range.
do not change the aircraft configuration, in other words don't mess with the flaps or the gear.

In simulators it's a wild ride. You can tell how far into the stick shaker range you are by the frequency of the shaker by feel and by its noise. The closer you get to an actual stall, the faster the stick shaker will go, so the idea was to keep the shaker on but not too fast. Since the airspeed will be varying rapidly, you have to do that with large pitch changes.
There was controversy back in the 1990s as to whether what we were trained to do was actually the best way, especially as to not changing the aircraft configuration and how deep to go into the stick shaker. Perhaps now the thinking is different.

I had a sim check where the check airman gave me a microburst in the turn to final over the potomac going into DCA runway 19.  Rolled level but wasn't quite aligned with the runway riding the stick-shaker with max available thrust.  I saw the RA clicking ever lower and my FO claimed it read "5" before positive rate was established.  I was absolutely sure I was going hit the ground and readying to stop ASAP to avoid overrunning into the river.  fun checkride.

I don't think the procedures are much different now. Put as much energy into the plane as possible so you have enough to stay above ground. Makes sense. If you change configuration like raising the gear, you will add more drag for several seconds, and this might be too much - after all, microbursts are a short-time event, so transition effects are important.

Is it then a better pratise to avoid situations like above, or do pilots have no otpion but to go through them.

@Firee When the safety of the aircraft is involved, pilots have the authority to do whatever they believe is safest. However, in the case of microbursts, it's not the case that you can know one is in front of you and are deciding whether or not to penetrate it. No pilot would do that. You don't know you're in a microburst until you're already in it. Then, of course, you have no option but to continue until you're through it. Microbursts are relatively rare. In 30 years of flying I never encountered one.

@Firee the only place you are going to get quick and reliable detection of an event like this is a big airport with anemometers around the airport perimeter.  These will also only detect wild wind differences once the event is in progress (the downdraft core precedes the surface boundaries) and unless the core is over the airport the wind signature wont be as clear.  Basically, unless you are somewhere you shouldn't be (under a well developed thunderstorm or virga) you may have no warning of a microburst event until the plane starts complaining that you are in one.

Terry & Casey: I think both your comments have answered my query, that its unlikely that a pilot would come to know about it beforehand.


text:
Why have different departure hall and arrival hall locations?
For most of the newly built airport terminals I've seen, the arrival halls are located on the lower level and the departure halls are on the upper level.
What is the benefit and disadvantage when:

Departure halls are above arrivals.
Arrival halls are above departures.
Arrivals and departures are on same level but in different places.

How do arrival/departure locations inside the airport impact the placement of airliners at the gates?

I'm pretty sure this is a dupe, but I haven't been able to find the original.

See http://travel.stackexchange.com/questions/49604/why-is-the-departures-level-always-above-the-arrivals-level.

@aeroalias, yup, that was it! OK, so it's not a dupe but it's been answered on SE.

I'm voting to close this question as off-topic because it already been answered at http://travel.stackexchange.com/questions/49604/why-is-the-departures-level-always-above-the-arrivals-level. Suggest migration or close altogether,

To be more specific, is this asking about the cases where there are actually separate airside arrivals and departure halls (i.e. like at ICN, HKG, etc.) or where there is only one airside hall, but spilt ground side arrivals and departures floors (like at most larger U.S. airports?) The reasons for these configurations are different, but are really probably mostly more on-topic at Travel.SE than here.

I'm not sure if 'most' airports have this design. I've seen both cases (i.e. departures below arrivals and the other way around). Atleast for small airports, a few reasons come to mind for having departures above arrivals:

If arrivals is on ground floor, the baggage can come easily from the aircraft. This reduces complexity, especially in small airports.
Most of the aircraft are boarded using jet bridge- makes sense if they are boarded from first floor (or above); of course, jet bridges can also be used for disembarking, so this is questionable.
The upper floor offers a good view to enjoy while shopping or eating. This maybe the reason as more time is spent in departure. Also, if the arrival is in ground floor, one can simply 'walk out' as no checks are there (international flights are a different case).

None of these arguments are convincing, I guess.

The baggage and jetway arguments combine to be quite convincing from a logistics standpoint in older terminals (where everyone enters and/or leaves on the ground floor): You check your bags when you enter the airport (at the ticket counter on the ground floor), and pick them up when you leave (at the baggage carousel), which leaves the upper level of the terminal for boarding/deplaning and security screening.  Newer terminals where departures enter on an upper level mess with that concept though. I suspect it helps with landside traffic?

@voretaq7 Most of the largish passenger airports I've been to in the U.S. have the ticket counters on the upper floor and just the baggage (and maybe car rental, etc.) on the lower floor. They usually have a separate drop-off area on a (car) ramp that is literally directly above the pick-up area. Usually road signs will direct traffic to either 'departures' or 'arrivals', with the former going to the upper ramp and the latter to the lower ramp (or the ground, as the case may be.) This is the case at Nashville, Tampa, and Orlando, for example.

@reirab It's the case in the new (and renovated) terminals at Kennedy too, and the new LGA terminal. I don't know if LGA is still using the old terminals but those had single-tier roadways & double-tier interior: Everyone entered/exited on the ground floor, departures dropped off luggage at the counter  and went upstairs for security and boarding, arrivals came downstairs for baggage claim & their taxi/car. The single-tier roadway created some traffic nightmares.

It has something to do about terminal design.
Equipements, handlings and airport services has to operate at ground level. That's where departures and arrivals at an upper level came from. There is not always the necessary room to lay everything at ground level. Though, thousand of smaller airports are not multilevel, mainly because they don't have that much passengers and cargo movements to justify the amount of investment to do for such extensions.
But why, when cost is not an issue, departures are usually above arrivals ?
Cost is always an issue. Pragmatic and practical reasons :
Elevators, speedwalks, escalators... works with electricity and must be maintained. The more you add those in the design, the more it costs. You can avoid them when you were able, by default, to bring most things at an higher level, and let them move with the help of gravity, or at least, reduce energy consumption by limiting the cases where something or someone has to go at an upper level. This doesn't explain why departures are usually above arrivals, but is the start of some explanations below...

as I said, luggages can be moved with the help of gravity. Let's assume departures are at ground level and arrival upstairs. Passengers's luggages go straight through checkings upon departure, then are loaded in the aircraft. Arriving luggages are unloaded from the plane, then goes upstair (move up). If it was the other way around, departing passengers's luggages goes downstair (move down). At this stage, the only difference is the move up/move down. When it comes about cost, it's better to move down only.
But, upon departure, you not only have to check-in. There are control checks, and in case of international flights, extra checks. That is available space concern. Plus, you add boarding area with benches/chairs, duty free, lounges, and other goodies that consumes space. Obviously, you ought to have a lot of available space for your terminal to have both departure flow and equipement services at the same level. That's why, on some terminals, arrivals are on the same level as those equipement services as they require less formalities than departures, and can be designed to fit on the same level as services. => departures on the upper level.
Parking lots. By design, it cost less to set parking lots at ground level. It's much more expensive to place them underground. While raising them above ground can be much more or less expensive than underground ones depending on the circumstances, it's usually assumed it would cost much more. Most frequent practice is to bring departing passengers in front of the checkings (few steps to do) then park the car. Reciprocally, arriving passengers has to bring their luggage on their own to the parking lot, as it's uncommon to let hundreds of car peacefully wait in front of arrivals. If most parkings lots are at ground level or underground because of design cost, it's more practical to align arrivals with parking lots, or at least, narrow the difference in levels, both reducing elevators or alike usage/congestion, and amount of effort required from the passengers to reach their vehicle. (also note that on some airports, you can't bring luggage trolleys outside the terminal, meaning, you have to pull your 40lbs bag all the way to the car)
It's better to wait in a roomy area than a narrow one. Think of the difference between waiting for boarding upon departure, and waiting for your luggage upon arrival. Even if it's usually wrong, it's assumed that you spend more time waiting for boarding upon departure, than waiting for your luggage upon arrival :P Well; in fact, to help you stay in a good mood and be open to spend some $$, departure areas are usually rooooooomy and beautiful ! You can only take advantage of that on the uppermost floor. People don't buy that much things upon arrival. They just want to have a rest, or get on time at their meeting or connection.

On a passenger's point of view, what matters is the distance one has to walk with that 40 lbs bag (even without) than anything else, no matter it's departure or arrival. Stairs are poor designs and not practical for trolleys. Elevators are known to be prone for traffic jams. The departure above arrivals is the design that best suits this requirement, and doesn't cost too much.


text:
Which operating systems do entertainment systems run on?
Which operating systems run the In-Flight Entertainment Systems on commercial passenger aircraft?

Most are Linux, a large number are Panasonics Ex3 which is a customised *nix.  I don't have reputable sources but will come back if I find some.  Android is taking over and I believe that some are Windows NT based.

There's no one operating system. Different vendors make different systems, which run different OSes.

There are so many different in flight entertainment systems running on different aircraft...I think you need to narrow this down a bit (particular systems in a particular plane with a particular carrier).  I'm marking it as too broad in the mean time.

@Simon, Android is also Linux. It just isn't GNU/Linux, but some other Linux-based embedded systems aren't either.

FWIW, On a recent Qatar flight (a brand new A330) my IFE screen and controller crashed and had to be rebooted. The screen was running some flavour of Linux, whereas the touch-based controller was Android (complete with droid logo on reboot)

@Jamiec An ongoing, religious and off-topic debate ;) But for all practical purposes, when reporting OS pentration, Android and Linux are treated separately.  Personally, with no GNU and using bionic instead I believe that Android is a distinct OS which is based on the Linux kernel. Developers certainly don't think of Android as a Linx distro.  It would be like calling OSX BSD.  Android => *nix is more accurate.

@Simon I think you @'ed the wrong person ;)

@Simon Linux refers only to the kernel, even though most think of the filesystem and utility programs as part of it when it's not. The rest of the OS is the "GNU" part. Android uses Linux.

According to this article, Linux is at least one of the main contenders in the space, Wikipedia also makes a lose reference to Linux and cites this article as the source. Here is some photographic evidence that its Linux as well. Although I'm sure it varies by maker it seems that Linux is at least one of the main players.


text:
How could it be possible to fully prevent wing tip vortex formation?
I know that winglets and swept wings are ways to reduce the vortex that are currently in use on many aircraft. Are there any aircraft design concepts out there that completely eliminate the vortex effect?

How about a GEV?

I had a comment on your other deleted question. Just posting it here. You could be interested in reading a gliding handbook. The FAA has very good manuals for learning aviation, e.g. PHAK, it provides a complete background for aeronautical topics. Also and this one.

@SteliosAdamantidis A ground effect vehicle would still produce wingtip vortices, they would just dissipate very quickly as they hit the ground (long before they could become problematic to something behind the ground effect vehicle).

Wingtip vortices are an unavoidable side effect of producing lift with an airfoil of finite length (thus having "wingtips"). The wing is designed first to have relatively high pressure underneath the wing and lower pressure above it. Second, air passing over the top of the airfoil is directed downwards as it leaves the wing (thus "pulling" the wing upwards by Newton's Third Law). These two characteristics of an airfoil combine to make air around the wingtip want to "spin"; high-pressure air below the wing will "leak" upwards over the wingtip, then be directed back downwards along with the rest of the air, creating the wingtip vortex:

Reducing the pressure differential between air above and below the wing will by definition reduce lift, because it is this pressure differential producing lift. By the same token, reducing the "downwash" off the back of the wing will reduce lift because the force creating the downwash is also lifting the wing. Therefore, conventional wings will produce wingtip vortices by definition.
Theoretical ways to reduce this vortex include:

Make it harder for air to "leak" upward over the wingtip. This is the principle behind the "winglets" you see on many airliners; they prevent air iunderneath the wing being able to move over the top of it, thus reducing the severity of the rotation of the vortex (which reduces drag, which saves the airlines fuel):

Even small aircraft, like this PiperSport light sport, often have little blended winglets to reduce vortex drag, saving as much available power as possible from the limited-horsepower engines of this class:

The A-10's wingtip has a similar idea implemented the opposite way; a slight downward curve to the wingtip holds the "cushion" of higher-pressure air under the wing, reducing the amount of leakage around the wingtip, which lowers the wing's stall speed and also reduces tip vortices:

Don't have a wingtip. "Nonplanar" wing designs, including ring wings, reduce (but do not eliminate) vortex drag compared to conventional wing planforms simply by not providing a single border between lifting and nonlifting surfaces:

This is a concept drawing, but the idea of a tipless closed-loop wing has been incorporated into production small planes, like this Belarussian small single:

Vortex drag is not totally eliminated because there is still a downwash behind the wings relative to the undisturbed air to either side of the plane, but the intensity of the vortex (and thus the energy lost producing it which counts as drag) is reduced considerably.
Reduce the pressure differential at the wingtip. This is a major way that swept, delta and double-delta wing planforms reduce tip vortices. Very simply, most modern wings on high-speed or high-performance jets have a smaller chord length and a thinner cross-section at the wingtip than the wing root. This provides less lift at the wingtip (and the wingtip stalls more easily and thus tends to stall first) than a straight wing with a relatively constant cross-section, but the reduction in pressure differential and downwash right at the wingtip reduces the two primary contributors to wingtip vortices:

Couldn't you reduce all lift at the wingtip.

You could, that's basically what blended winglets do. But, there's a balancing act, increasing surface area increases drag, and a flat wingtip is pure drag, no lift.

@Ethan, the air used to produce lift is accelerated downwards (principle of action and reaction a.k.a. Newton's 3rd law) and the air not used to produce lift is not and that is the vortex. All you can do is prevent high speed core at the tip by making the transition from making lift to not smooth.

@JanHudec I do have a couple ideas in mind that can prevent wing tip vortex. Haven't tested my theories out though.

@Ethan, unless you have found a way to avoid 3rd law of motion, sorry, no,  they won't work. And since principle of action and reaction follows directly from conservation of momentum and conservation of momentum from elementary symmetry of space, I really doubt you did.

@JanHudec Well my theories still form the vortex, but stop the vortex before it reaches the wing tip. None of my theories avoid the 3rd law of motion. I have tested one of my theories on a paper airplane and saw a longer range in the plane. Not saying it worked, but it reduced it.

Forget the tips, they have nothing to do with it. The air behind the wing must be moving downward, because it acted on the wing with upward force, so the wing acted on it with downward force and that accelerated the air; and since there is nothing else to cancel the acceleration, the air will, necessarily be moving. And the air outside the wingspan won't. So we have two streams that move relative to each other and that is a vortex. Its vortex line flows from the wingtip, so its called a wingtip vortex. But the cause is not the tip, the cause is the lift and laws of motion.

@JanHudec don't forget the idea of putting propellers at the wingtip, spinning against the vortex direction. Real-world example: https://en.wikipedia.org/wiki/Vought_V-173 Dunno if the vortex was perfeclty, totally eliminated, but they were able to make a very low-aspect ratio wing with plenty of lift and maneuverability.

@Ethan See above comment. I found out about it from asking a similar question: http://aviation.stackexchange.com/questions/24525/can-wing-tip-vortices-be-reduced-eliminated-with-a-rear-facing-propeller-near-th

@DrZ214, that does not eliminate anything, just increases the effective wing span. The propeller slipstream rotates so the air moves down outside of the wings. Since the air of the propeller span does not move, the "wingtip" vortex moves to the tip of the propeller.

Wingtip vortices are unavoidable in a three dimensional lift producing wing. The vortices are a result of the pressure difference that causes the lift. The only ways to eliminate the vortices are,

Make the wing two dimensional- i.e. make its span infinite, like an airfoil.
Don't produce lift.

Unfortunately, neither of these help us much.
Some points to note are:

Winglets and other wingtip devices reduce the intensity of the vortices, not eliminate them.
'Closing' the wing or wing tip does not eliminate the trailing vortex wake or induced drag. From aero.stanford.edu:

Wings that form closed loops ... do not eliminate the "tip vortices" or trailing vortex wakes even though the wing has no tips.

The figure below shows the CFD simulation of wings with different wingtip devices.

Image from Aerodynamic Efficiency Study of Modern Spiroid Winglets, by Tung Wan Hung-Chu Chou Kuei-Wen Lien, Figure numbers removed for proper fit.

I wouldn't say unavoidable I have a theory about how to eliminate the vortex, but haven't tested it out yet.

@Ethan, unavoidable. Totally. Read also the answers to the duplicate question.


text:
Why do older airplanes have a wing on both the top and the bottom of the aircraft?
The first airplane by the Wright brothers had a double wing. This concept continued through World War 1 and into the 1930s - why was this concept popular?

Not all older airplanes are biplanes. See the Bleriot XI that crossed the channel in 1909.

@Ethan Are you asking about biplane vs monoplane or about the wing configuration of biplane (one low wing and one high wing)?

Mainly for strength. Up to about 1920, wings used very thin airfoils and were mainly made of fabric stretched over a wooden frame. The main spar was too thin and would have been prone to bending if it had had to support the whole weight of the aircraft in a single span. Only by using two wings, the upper as the compression member and the other as the tension member of a truss, the needed strength was possible.

Airfoil thickness comparison by D.R. Kirk, Florida Institute of Technology (picture source)
In your photo, you can just about see the wires running from the top wing (near the outboard struts) to where the bottom wing joins the fuselage. These wires take most of the strain in flight, spreading the load on the wings. It's the same principle as using triangles in the construction of electricity pylons or bridges.
Monoplanes of that time needed even more wire bracing. Look at the picture of a replica of the Etrich Taube, a very popular plane of the pre-WW I period.

Etrich Taube in flight. Note the truss below the wing and the many wires keeping it in shape (picture source).
The shorter wingspan of a biplane also reduces the load on the wings. Biplanes tend to have a lot of lift (for their size) because of the large wing area, but also a lot of drag, so they're quite inefficient overall. There were also triplanes, with three mainplanes for even more lift in the same wingspan. WW I pilots demanded the highest rate of roll, which could best be achieved with biplanes.
In the time between the World Wars, the use of high-strength aluminium for aircraft and the monocoque airframe both allowed airframes to become stronger. This meant that monoplane designs became more practical, even though the older aviators didn't believe a monoplane could be strong enough. As soon as monoplanes were practical, the improvements in efficiency and flight performance saw them replace bi- and tri-planes.

So they needed 2 wings to hold up the aircraft better, but in ww2 the had stronger wings to hold the body of the aircraft better. Is this basically what you are saying.

I'm not saying you are wrong but there are plenty of wooden monoplanes that could do with being explained.

@RedGrittyBrick The Goose had wings about ten feet thick and only flew once. It doesn't really need much explaining.

@DanHulme: And what about the Mosquito, the "wooden wonder"? Your explanation is half right, but you omit that airfoils initially were very thin, so they could not carry the needed structure internally. Using steel, btw., would had not made a difference.

I understand how the upper wing experiences compression(because the flying wire is at an angle),but what I don't get is how and why the lower wing of a biplane is the tension member?..@Dan Hulme @Peter Kämpf

@JessicaHam: Think of a wooden beam. Support it on its ends and add a load in the middle. It will bend, stretching the lower parts and compressing the upper parts. Now remove all the wood in the middle and leave only the upper- and lowermost layer. Replace what has been taken away with struts and wires. That is what a biplane designer does – he creates a strong yet lightweight beam for high bending loads.

In the early days, it really looks as if people hadn't really worked out what worked best

Horatio Frederick Phillips, 1904 Multiplane. Longest flight 50 feet (15 metres)

Gianni Caproni, Ca.60. 1921. Crashed on second flight
A wire-braced box girder structure can probably provide better strength and rigidity than a single, and much longer, wooden spar.
An advantage of multiple wings is that you can have shorter wings, and this can improve maneuverability in combat

Fokker triplane. 1917
Maybe a better question is why people opted for only two wings when they could have had twenty?

Again, your answer is the best of the pack. It's too bad that so few people can appreciate this.

"honey, come to dinner." "In a minute, soon as I troubleshoot this 'flaps disagree' indicator..."

What is the airplane name of picture 1 and picture 2? Were they really fly?

@AirCraftLover: I have added captions. They flew briefly. See recent models of multiplane

Thank you for the info.

If we see from the Horatio Frederick Phillips (also from the model on the Youtube video) and the Gianni Caproni's multiplane, the wings are stacked exactly one above another. Why they were different from the triplane? As I can see that triplane, the stagger is leaned forward. Why have to be so if it can be stacked like the multiplane?

@AircraftLover: That would make a good new question  -- except it has already been asked

Sure. I will ask it.

You may have enough background to answer why there were no tailless biplanes after the first batch.  https://aviation.stackexchange.com/q/67441/31425.

The main reason for having multiple wings in the initial years of the aviation was the lack of availability of materials with sufficient strength.
The main advantage of the biplane is that the wings could be shorter for a given lift. During the initial stages of aviation, the materials available for manufacture of aircrafts were timber, doped fabric etc. The structural members made of these did not have enough strength, limiting the wing size.
The early aircraft also had less powerful engines, resulting in lower speed. In this case, the lower stall speed of the biplanes was advantageous.
The biplanes had very good maneuverability (better roll rate) compared to the monoplanes. This is the reason triplanes were developed during the WWWI.

"SopTri3" by http://www.earlyaviator.com/archive/a/images/tripe_peggy.jpg. Licensed under Public Domain via Commons.

However, with the advent of better materials like Aluminum alloys, the aerodynamic disadvantages of biplanes became apparent and monoplanes rapidly became the preferred design choice.
As the material strength improved, the aspect ratios have become ever bigger, with civil aircrafts facing operational (airport) limitations rather than strength limitations in wing length.
However, the biplanes are still used. The better maneuverability and low stall speed make them popular aerobatic aircraft.

aeroalias ever since you started on this website you have had the best answers. So keep giving other people good answers too. Thanks for answering my questions.

@Ethan, don't be afraid of waiting a few hours, or even a couple of days to select the answer that works best for you. You don't always have to accept the first answer. (Not saying that was the wrong choice this time, but something to keep in mind.)

To explain my downvote: The materials were not the problem. At the speeds possible 100 years ago, a thick wooden wing would have had plenty enough strength. No, it was the prevalence of very thin, highly cambered airfoils which simply did not have the space for an internal spar. Sometimes your answers are spot on, but sometimes you are just guessing. This time you guessed wrong. Also look at @RedGrittyBrick's answer for some valid reasons.

@PeterKämpf While a thick wooden wing can have enough strength it's too massive to be used in those low powered planes. Note: wooden strength relates more less to it density so I cannot see a massive ebony or hickory wing working here

Actually, there were quite a few monoplanes around even in the early days:

1909: Bleriot XI crosses the English Channel
1910: The Fokker Spider flies around the Haarlem church tower. Young Anthony is 20 years old.

1912: Deperdussin Monocoque, the first design with a stressed-skin fuselage.
1913: Morane-Saulnier H

1915: Junkers J1
1915: Fokker E II: Max Immelmann

1915: Fokker E III
1918: Junkers D.1

1919: Junkers F-13
1921: Fokker F.III

1924: Fokker F.VII
1926: Ford Trimotor

1933: Boeing 247
1934: DC-2
All images from Wikipedia

Nice collection! A good addition would be the Deperdussin racer from 1912, one of the first designs with a stressed-skin fuselage.

@PeterKämpf Done.

Antonov three. Tee hee!

Mind that that's an F.VII-3M, the 3 engine derivative of the F.VII. It was license built by Ford who developed it into their Trimotor.

Even though some early aircraft were designed with cantilever wings which did not need wire supports, there were still a number of reasons why the wire truss biplane design remained in use for a long time. Like any new technology, development takes time. Fokker designer Reinhold Platz' designs during WW1 is a great example. His thick wing, box spars were a sound design, yet glue was not necessarily very strong, and/or the skilled labor for assembly was poor which created problems, and deaths. The wood is not the problem. High quality spruce actually has about the same strength to weight ratio as aircraft aluminum, though the AL is more isometric, wood proved itself worthy of aircraft construction when used correctly. When the Fokker box spars failed, word got around fast, and generated fear and loathing by Pilots who liked to know their wings would not fold. Pilots didn't dislike biplanes, and wanted "what we know already works". The triplane had box spars and were true cantilever wings without braces and struts, yet pilots veered away from it in great fear, and refused to fly it. So, Platz added the interplane struts to tie all three wings together at the ends purely for the impression it made on pilots. Wires were however eliminated. The Fokker D8 monoplane had a true cantilevered wing which also had some quality issues early on which of course usually ends in a death, and pilots didn't like that. Wired monoplanes had no advantages, High drag, lower lift and weaker than biplanes. Pilots, passengers and investors liked biplanes. Designers eventually proved their knowledge to pilots and to the public, but like anything else there is an enormous psychological element- A not altogether invalid fear that slowed progress. Military people are the worst at accepting new ideas, with a long history of accepting only the tried and true, with no real desire to put pilots at unnecessary risk. Plus there were two distinct advantages to biplanes, adequate lift with shorter wing span, plus the associated high roll rate, and strength was, and is- very good, they were in fact hard to beat.

Structural strength was not the main reason for early bi-planes. Otto Lilenthal did his glider experiments in a monoplane like structure. The wright brothers were following his research but they wanted to create powered flight. The earliest engine didn't have sufficient power. A Biplane could generate more lift at slower speed. A monoplane couldn't. That was the primary reason for choosing a biplane design.

By the 1930's, powered aircraft were much faster than Lilienthal's gliders.  When we revisited underpowered aircraft (human- or solar-powered), we reverted to monoplanes.  So the claim that biplanes make more lift at slow speeds is both false and irrelevant.  -1.

The reason a biplane could generate more lift at lower speed was because it had greater wing area for the same weight. That was only possible because it was inherently stronger than a monoplane of the same weight.

It is not so that wood and fabric were not 'strong enough' materials to build monoplanes and longer wings.  (That's a plural, by the way. it doesn't need an 's' on the end that more and more people nowadays stick in.) Peter Kampf rightly draws attention to the highly advanced wooden monocoque fuselage structure of the Deperdussin of 1912 and a glance at the long-winged wooden gliders of the Wasserkuppe a little more than a decade after WW1 shows how erroneous the idea is. The two features already mentioned governed the design and structure of the multi-wing aircraft: thin aerofoils used on early aeroplanes (knowledge of aerofoil physics was, understandably, also pretty thin at the time) and the desirability of a high rate of roll. Always described as 'flimsy' by journalists, these WW1 machines were anything but. The biplane truss is immensely strong.


text:
What are the basics of flying non-revenue?
I've never worked for an airline, but I know that one of the benefits of working in the industry is that you can fly for free or almost free on a space-available basis.  I realize that every airline is slightly different, but what's the general procedure for this?  To provide a concrete example, let's say I live in New York and want to non-rev to Miami tomorrow.
How would I know if a space is available?  How far ahead will I know that I have a seat?  Can more senior employees beat me out or is it first come, first served?  Do I need a boarding pass?  If not, how do I get past the TSA checkpoint?
In general, how does flying non-rev work?

There are a few classes of Non-rev travel.

Jumpseating
Online pass travel
Interline pass travel

Jumpseating
This is open to pilots and flight attendants, air-traffic controllers (not sure if this is currently true, but it used to be), certain company and government officials (e.g. the FAA and others).
In general pilots can only occupy pilot jumpseats and flight attendants can only occupy flight attendant jumpseats.  I know of cases where pilots can sit in the FA jumpseats but this is largely a topic of the Pilots and FA Union contracts.  In general pilots can jumpseat on any other 121 carrier and some 135 carriers.  Note that jumpseating does not necessarily mean you'll be in the cockpit, as current regulations stipulate you should not sit in the cockpit unless there are not seats in the cabin available.
If you list for a jumpseat you will generally be the lowest priority for a cabin seat, boarding after all other non-rev travelers and then in a specified priority order jumpseaters will fill up the cabin and then the cockpit jumpseats until space is exhausted.
Jumpseating is the most versatile of the options, but the most limited in who can use it.  It is good to keep in mind your jumpseat agreements probably allow jumpseating on cargo 121 carriers in addition to passenger 121 carriers, so keep Fedex, UPS, Atlas, etc in mind when travelling.  Your spouse/kids won't be able to join you though.
Online Pass Travel
The specifics of this travel depends on your airline, but I will speak in general terms.  Pass travel is usually available to the employee, the employeers domestic partner or spouse, the employees minor children and the employees parents.  There is usually also a program for a limited number of passes for friends (buddy passes).
Pass travel is usually limited to your airline those you have a business relationship with.  For example a regional carrier can usually pass travel on the legacy carrier they contract with.  A legacy carrier can usually pass travel on its regional contracts.  If a regional contracts with multiple legacies, you sometimes only pass travel on the legacy system of the legacy you perform flights for.  It can be complicated.
There is always a defined priority for pass travel.  In general employees ride before spouses/parents/children who ride before buddy passes (except when travelling together, you may all share the priority of the highest priority pass on the itinerary).  When multiple companies are involved, the priority levels may reflect a higher priority for the company operating the flight employees than the legacy/regional contract carriers.
Again, this will vary greatly from company to company and you will learn the specific rules that apply to you when you are hired somewhere.
Interline travel
These passes are known as ID75, ID90, etc where the number is the percent discount from a full fare ticket.  You can generally buy passes for yourself, your spouse/domestic partner and your children.  No buddy passes.
To get an ID90, you visit your travel office and request an interline ticket for a specific city pair.  For example you can buy an ID90 for Rome - NYC.  You get an old school paper ticket for that route and no specific airline.  When you want to use the ID90, you present it to the ticketing agents for the airline you want to ride that flies the route you want to fly.  They will in turn issue you a non-revue standby boarding pass.  These are generally lower priority than online pass travel when it comes time to board.  These tickets are fully refundable, so they are often purchased as backups to get home when travelling in case you cannot get back with online passes.
How it all works
If you are the employee, you have a company ID and use this to get through security.  If you are the spouse/partner/child/buddy, then you will print out a standby boarding pass and use that to get through security.  You can check bags, which will get tagged standby and should board if you do.  It can be good to remind the gate you have bags checked when you get a seat.
If you are flying on your own carrier or under your legacy brand, you generally have computer access to the passenger boarding totals.  These can often be hard numbers of people booked, checked in and standbys listed.  You can use this to gauge the likelihood of getting on a plane.  If you are travelling offline then a carrier might have a phone number available but they may only tell you "it looks good!" or "it looks bad" and will not divulge numbers.
Once you are at the gate, you play the waiting game.  You will have a boarding priority which may look something like this for a legacy carrier flight:

Legacy pilots
Legacy flight attendants
Legacy retired pilots/FA
The legacies regional contract pilots / FA
Legacy spouse/parents/domestic partner/kids
The regional contract spouse/parents/domestic partner/kids
Codeshare pass agreements
Buddy passes
Interline passes
Cabin jumpseats
Cockpit jumpseats

This will assuredly vary based on who you work for and who they contract with, and is just to illustrate the complexity of the priority structure.  Within each category listed above there is an internal priority that for many is based on date of hire, others based on time of listing.  Again, this varies.
After all the confirmed revenue passengers board, the gate will board the revenue standby passengers, and once they are on and the number of empty seats is known (misconnects, people not in the boarding area 15 minutes prior to departure, etc), they will board according to the non-rev priority.  When the cabin is completely full, no more non-revs may board except for the jumpseaters, who may then fill the available jumpseats.
Putting it together
If you wanted to non-rev from NYC to Miami tomorrow and your company flies that route, then you just wake up in the morning, list yourself, take a look at the boarding totals and head to the airport.  If you don't even want to do that and you are a pilot, you just go down to the airport, find a gate with a plane headed to Miami and list for the jumpseat on that plane.  If the many rules of priority work out, you get on and you go to Miami.
Some considerations
Pass travel often has a dress code.  Don't show up in a tshirt and shorts unless you know you are allowed, you can be denied boarding.  Make sure all of your pass riders are behaving themselves.  This is especially important to impress on anyone you give a buddy pass, as when they screw up as a pass rider, you can have your pass travel suspended as punishment.

The exact procedures, policies and systems are going to vary depending on the airline, but the following is based on my non-rev experiences with a particular airline.
Booking your flight and checking loads
First up, most airlines will have some sort of system whereby you can log in and view flights and availability. This system will work similar to booking a flight through the normal system, but will also give an indication of availability. For the system I am familiar with, it shows a breakdown of the number of seats available for sale in each cabin, and the number of seats booked in that cabin (note that this does not reflect the number of actual seats, as airlines often overbook). It also lists any other staff members who are listed for standby on the flight.
With the list of staff on standby, these are organised by priority. Priority is based on role at the airline, number of years served, and type of standby ticket .
Based on all the above, you can get a rough overview of the loads for the slight, and how likely you are to get on-board. However, there are a few things to bear in mind. Firstly, a lot of people tend to book last minute, particularly on key business routes. Also, if you are flying from an airlines hub, there may well be a number of passengers that miss a flight, and get transferred onto the flight you are looking at.
In short - you never know until you get to the airport, even then it's a waiting game.
At The Airport
Once at the airport, you check in as normal. Depending on the loads of the flight, they may issue you a seat and confirm you on the flight (if it's completely empty), but more often than not, you remain on standby. The procedure here varies depending on whether you are at your airlines hub, or down route. Typically at the hub, there is a dedicated staff travel waiting room, where you can go and wait for information on your flight. At a non-hub airport, you are normally checked in and your bags tagged as "standby", then you are issued a boarding pass (with no assigned seat) to get you through security. You then wait at the gate for information.
The Waiting Game
Unless the flight is completely empty, you often wont know if you are on the flight until check-in closes (approx 30-40 minutes before departure). At this point the flight is no longer open for transfer passengers, and they know how many passengers booked on the flight have turned up. At this point you will either be called to go to a check-in desk, or given a boarding pass at the gate. Although once check-in closes, there is varying amounts of paperwork to complete, so if you are waiting at the gate it might take another few minutes before everything else is sorted and they assign you a seat.


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There are some types of landings, can you, please, clarify them? Or even add some other types
I'd like to have explanations of these landing descriptors - what do they mean?

deep landing
bounced landing
hard landing
nose pointed down landing
technical landing
"smooth" landing
belly landing
landing gear up landing
partial landing gear up landing

They are not types, just a landing with a description, like in "good landing".

I bet you understood my doubt, and I appreciate your answer, thanks a million.

From European Action Plan for the Prevention of Runway Excursions 1.0, Appendix E- Aircraft Operators:

Deep landing – a certain distance behind the glide slope touchdown point
Short landing – touching down before the glide slope touchdown point

Belly landing (or pancake landing or gear-up landing)-  The aircraft lands without its landing gear fully extended and uses its underside, or belly, as its primary landing device.

"Thunderbolt II 080325" by U.S. Air Force photo/Brad White - Official USAF Website 1. Licensed under Public Domain via Commons.
Partial gear up landing- One/or more of the landing gears fail to extend. Similar to belly landing, but in that case, all the gears fail usually.
Hard Landing- The aircraft impacts the ground with a greater vertical speed and force than in a normal landing. The actual speed varies from aircraft to aircraft. This kind of landing usually requires some inspection afterwards.
Nose pointed down landing- Aircraft usually land at a positive attitude (i.e. with nose pointed up). However, in some cases (usually due to some problem), the landing can be carried out with nose pointed down. This is normally in violation of standard procedures.
Arrested landing- Usually done in carriers; an arrester cable is used to bring the aircraft to stop.

"FA-18 Trap". Licensed under Public Domain via Commons.
Smooth landing- If the landing has no problems like given above, it can be called a smooth landing. It is subjective though.

Deep landing – a certain distance behind the glide slope touchdown point Should this be "beyond" the td point? I read "behind" as the same as short landing

@TomMcW Yes. It should be 'beyond'. it would have made things more clear.

I think the answer is interesting, but I'd suggest to make two categories: known landing techniques (e.g. gear up landing) used in some circumstances that can be explained, and cases that are unexpected and the consequence of how the landing was conducted or how the environment played a role (e.g. hard landing / nose down landing after wind shears). The question itself could be improved to ask for how much in such or such landing is intended or a mistake). Just a suggestion.


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Do flaps have a maximum speed limit but spoilers don't? Why?
This related question on Aviation SE describes how in most aircraft spoilers (flight spoiler mode not ground spoilers) are deploy-able at any speed But flaps on the other hand mostly have a V_FE above which they cannot be deployed.
I assume the speed limit imposed on using flaps is due to structural reasons? I am just wondering why spoilers do not have a similar limit? Looking at a spoiler photo in the deployed position it looks like the spoiler mechanism ought to be under a lot more stress than the flaps. Am I wrong here?
Or are spoilers reinforced to a degree out of need that is not felt necessary for flaps? i.e. There can be a legitimate need to deploy flight-spoilers at a speed higher than for flaps.

Spoilers reduce lift potential when deployed and increase drag. This makes them doubly helpful at high speed: Not only will the aircraft slow down, but the pilot has less of a chance to break something important when they are deployed.
Contrast this with flaps: They create only a moderate drag increase, but vastly increase the potential lift. If they would be deployed at high speed, the pilot (or an unexpected gust) could easily overstress not only the flap structure, but also the whole wing. Extending them at high speed is suicidal.
Therefore, some regulations demand that spoilers must be useable even beyond the maximum speed. For gliders, where the spoilers are called airbrakes, JAR part 22 says in AMC 22.143:

Compliance with 22.143(a) should include the extension of airbrakes at
  speeds up to 1.05 VNE. The time to extend airbrakes should not exceed
  2 seconds.

For airliners the regulations are more lenient, and the manufacturer is free to limit their use as he sees fit. However, it is good practice to allow spoiler extension over the full speed range.

In typical modern airliners are there airbrakes independent from the spoilers?

@curious_cat: No. What is called airbrake in gliders is called spoiler in airliners.

@curious_cat: BAe146/AvroRJ has an airbrake on tail that is not a spoiler (spoiler implies it is on the wing and interferes with lift), but it is an exception.

@JanHudec Very interesting. I had never heard of this config. So many different ways to design an aircraft, eh.

@curious_cat: This is not so unusual. The Fokker 70 and 100 uses it as well, as did older designs like the Dornier 217.

Spoilers are used to shed energy when the aircraft is too fast or too high. That means they need to be used when the aircraft is flying fast and are designed accordingly.
On the other hand flaps are used to increase lift and when flying fast, there is plenty of lift, so there is no point in using flaps at high speed and therefore they are not designed for it.
Additionally flaps on large aircraft are more complicated mechanically. Spoilers only rotate around their forward edge, but flaps on large aircraft extend to the rear in addition to rotating downward.

OK but this does not answer the why part of the question. Normally both of these surfaces will experience roughly the same aerodynamic forces. But what is the difference that makes spoilers more powerful? Complicated == fragile for flaps is not an argument by itself. One might argue, "it's more complicated to be more durable".

@Stelios, the answer addresses why. Because the difference in what is useful. Perhaps I should delete the third paragraph; it's mostly irrelevant to the answer!

@SteliosAdamantidis Why are the forces that spoilers experience the same as flaps?  Flaps have a greater surface area and should experience much higher forces for the same airspeed.